1. Field of the Invention
The present invention relates to a semiconductor integrated circuit including a bipolar-type flip-flop circuit, and, particularly, to a technique to prevent erroneous operation of a flip-flop circuit due to external noise.
2. Description of the Related Art
Bipolar-type flip-flop circuits are used in various applications. In this description, here, there will be described a specific application of a flip-flop circuit to a control circuit used in an apparatus installed in a car or a motorcycle. In this application, the flip-flop circuit is often affected by ignition noise due to sparks of an ignition plug.
FIG. 9 is a circuit diagram illustrating a conventional semiconductor integrated circuit called a flip-flop circuit for use in a control circuit of an antilock brake system (hereafter referred to as an ABS) installed in a car or a motorcycle. In this flip-flop circuit 10 shown in FIG. 9, a reset operation takes precedence over a set operation. The flip-flop circuit 10 comprises: constant current sources I3, I6, I7 (I3, I6, I7 are also used to denote the values of currents supplied by these current sources); bipolar transistors (hereafter referred to simply as transistors) Q3, Q4, Q7, Q8, and Q9; and resistors R3, R4, R5 and R6. In FIG. 9, there are also shown a set terminal Set, a reset terminal Reset, an output terminal OUT, a power supply Vcc, and ground GND. In this flip-flop circuit 10, the flip-flop circuit is set or reset by a combination of transistors Q3, Q4, Q7, and Q8 to provide a Q signal and an inverted-Q (denoted by Q with an overline in FIG. 9 and other figures) signal. The Q signal is inverted by an inverter composed of the transistor Q9, and output via the output terminal OUT as an inverted-Q output signal. Resistors R3 and R4 are balance resistors for equally supplying the Q signal to bases of both transistors Q4 and Q9. Similarly, resistors R5 and R6 are balance resistors for supplying a set signal equally to the base of the transistor Q3 and the base of a transistor (not shown) in another block.
This circuit is used as one of the flip-flops included in a circuit that generates a signal when a failure is detected in a control motor (not shown) of an ABS. This signal is sent to a controller such as a microcomputer (not shown) responsible for control of the entire ABS so as to stop the antilock brake control and switch the control to a normal brake control mode. The set terminal receives for example, a failure signal generated by a sensor (not shown) for detecting overheating of the control motor or by a circuit (not shown) for detecting an overcurrent of the control motor. In response to this failure signal, a signal is provided to the microcomputer of the ABS via the output terminal OUT so as to stop the antilock brake control. One flip-flop circuit of this type is provided for each of failure signals of the overheating sensor of the motor, overcurrent detection circuit, etc.
The flip-flop circuit shown in FIG. 9 operates as follows.
In the following explanation, there will be shown an example in which the flip-flop circuit receives a failure signal from the overcurrent detection circuit for detecting a failure of the control motor of the ABS. Therefore, it is assumed that the set terminal is connected to the output of the overcurrent detection circuit, and the output terminal OUT is connected to an input of microcomputer of the ABS.
In a normal state, when no failure is detected, the set terminal connected to the overcurrent detection circuit is at an L-level. Just after the power supply of the system is turned on, a reset signal having an H-level is applied to the reset terminal. After that, however, the reset terminal is maintained at an L-level. In this state, transistors Q3, Q4, Q8, and Q9 are in an off-state, and the transistor Q7 is in an on-state. As a result, the Q signal is at an L-level and the output terminal OUT or the inverted-Q output is at an H-level. That is, in a normal state, an H-level signal is applied to the microcomputer.
If the overcurrent detection circuit detects an overcurrent flowing through the motor, an H-level signal is applied to the set terminal. As a result, the transistor Q3 is turned on, and thus the inverted-Q signal falls down to an L-level, and this causes the transistor Q7 to be turned off. As a result, the Q signal goes to an H-level, and transistors Q4 and Q9 are turned on, and thus the inverted-Q output at the output terminal OUT goes to an L-level. In this way, a signal indicating that a failure or an overcurrent has occurred is sent to the microcomputer. In response to this signal, the microcomputer stops the antilock brake control and switches the control to the normal brake control mode.
In the flip-flop circuit configured in the above-described manner, when a huge surge (ignition noise in this case) occurs on the Vcc power supply line or the GND line, a current flows for a short time through the parasitic capacitance of a semiconductor substrate (described later) into the bases of the transistors Q3 and Q4, which sometimes causes the flip-flop to operate in an erroneous manner (to perform an erroneous latching operation). This erroneous operation will be described in more detail below.
FIG. 10 is a cross-sectional view of the semiconductor substrate 100 on which resistors R3 through R6 shown in FIG. 9 are formed as p-type diffused resistor, while only one of those resistors is illustrated in FIG. 10. In FIG. 10, reference numeral 1 denotes a resistor acting as one of resistors shown in FIG. 9, that is formed in an n-type epitaxial layer 3 (called an island) surrounded by p(p+)-type diffused region 2 called an isolation-diffused region. In an actual integrated circuit, a plurality of resistors 1 are formed in this epitaxial layer 3. The epitaxial layer 3, in which the resistor 1 is formed, should be maintained at a stable voltage higher than the voltage of the resistor 1 so that the resistor 1 is reverse-biased thereby isolating the resistor 1. For this reason, the voltage of the epitaxial layer 3 is generally fixed to the power supply voltage Vcc. Reference numeral 3a denotes a terminal for connecting the epitaxial layer 3 to the power supply.
While a reverse bias voltage is applied between the resistor 1 and the epitaxial layer 3 for isolation, there are still small parasitic capacitances 4 as represented by broken lines in FIG. 10. These parasitic capacitances correspond to capacitors 4a-4d of the flip-flop circuit shown in FIG. 9. In general, when a pn junction is reverse-biased, there is a junction capacitance acting as a parasitic capacitance. If the fluctuation of the power supply voltage Vcc or the GND voltage is large, these parasitic capacitances 4 cause the flip-flop circuit to operate in an erroneous fashion.
Semiconductor integrated circuits installed in a car or a motorcycle are affected by ignition noise due to sparks of an ignition plug. FIG. 11A illustrates a waveform associated with the inverted-Q output at the output terminal OUT of the circuit shown in FIG. 9, in which erroneous inversion due to ignition noise is shown. FIG. 11B illustrates a waveform associated with the power supply voltage Vcc subjected to ignition noise. In FIG. 11B, V represents a voltage swing of 10 V and T represents a time period of 4 .mu.s. As shown in FIG. 11A, the inverted-Q output is inverted from an H-level to an L-level due to ignition noise. That is, if the power supply Vcc or the GND is subjected to huge surge noise such as ignition noise, a current flows for a short time through parasitic capacitances of the semiconductor substrate into the bases of the transistors Q3, Q4 (refer to FIG. 9), which sometimes results in erroneous operation.
It can be understood that the erroneous operation occurs as follows.
(a) In a normal state, both set and reset signals are at an L-level, and the Q signals is at an L-level and the inverted-Q signal is at an H-level. Thus, the transistors Q3, Q4, and Q8 are in an off-state, and the transistor Q7 is in an on-state.
(b) In this state, if external noise such as ignition noise enters the circuit, fluctuations occur in the power supply voltage Vcc or the GND voltage.
(c) As a result, a current flows through the parasitic capacitance 4a or 4b from the power supply Vcc to the resistor R3 or R5 as shown in FIG. 9. Thus, a current flows into the base of the transistor Q3 or Q4. If the amount of this current reaches several tenths or several hundredths of the collector current of the same transistor, the transistor Q3 or Q4 can be turned on. For example, if I3 is assumed to be 50 .mu.A, then a current of several .mu.A flowing into the base of the transistor Q3 or Q4 can turn on the transistor.
(d) In the above-described manner, the flip-flop circuit is set, and inverted-Q output is inverted from the H-level to an L-level. While this inversion is not due to a true set signal, a signal indicating an event of a failure is sent to the microcomputer of the ABS. As a result, the ABS stops the antilock brake control, and switches the control to the normal brake control mode.
In the above example, erroneous set operation of the system has been described. However, the system may also be reset erroneously in a similar manner.
What is a really serious problem here is that a transistor is for example turned on from an off-state due to noise and latched in this on-state. However, fluctuations that occur for a short time without leading to latching do not bring about any serious problems.
As described above, conventional semiconductor integrated circuits including a bipolar-type flip-flop circuit configured in the above-described manner have a problem that external noise can bring about erroneous operations.